Gravitational lensing

According to the theory of General Relativity, a massive object distorts the space around it. For example, the distortion in space due to the the Sun makes the planets go around it. When light travels through such a distorted space, it also gets deflected.

Therefore, in general, the gravitational field generated by a mass distribution will distort the images you see of background sources, pretty much like the hot air above a road distorts the view behind it. This is in fact a good way to think about this phenomena: it is as if space has an index of refraction which varies in space, because the gravitational field also varies in space. (Whereas it is of course the varying temperature that causes the refractive index to vary above the hot road.) This effect is called `gravitational lensing', although `gravitational miraging' might be a better description since the distortion is more `blurring' (like the hot air) than focusing (like a proper lens).

A very good introduction to the subject is the recent review by Joachim Wambsganss^12.1. Scientists are not very good at reconstructing the history of a subject, but Joachim claims that a German physicist in 1804 was the first to suggest that gravitational lensing might be observable, for example in terms of the deflection of star light passing close to the Sun.

There are several possible observable effects of gravitational lensing: (1) deflection: the image position is displaced from the source position, (2) amplification: the image may be become brighter or fainter, (3) distortion: if the source is extended (for example lensing of a galaxy by a galaxy cluster), then lensing may distort the image, (4) multiple images: in general a single source will be imaged in multiple images. All these effects have been observed.

In general, the deflection and amplification in real systems is usually very small, therefore our view of the Universe is not greatly distorted by gravitational lensing. But occasionally, some objects are lensed quite strongly. We'll start by deriving a very simple - but unfortunately not quite correct - expression for the deflection angle, but also state the correct result. Then we'll be armed with enough understanding to look at some applications of lensing, and you'll realise the tremendous potential of this technique, on a really wide variety of fronts, from constraining the nature of dark matter, via measuring the masses of galaxy clusters, to detecting planets and constraining models of stars.